This paper develops a mesoscopic approach to palaeostress estimation based on applying a new “fibre-loading” type theory to boudinage structure. The theory implies that, if boudinage develops by a sequential fracture and separation process, the distribution of boudin aspect ratios should be controlled by a critical ratio below which the tensile stress cannot exceed the tensile fracture strength of the boudinaged layer. Simulations based on distributions of tensile fracture strength and elastic moduli (derived from published experimental work) allow a distribution of boudin aspect ratios to be predicted. This compares very favourably with the observed aspect ratio distribution of quartzite boudins within Lower Carboniferous slates at the southern margin of the Culm Synclinorium, southwest England. Hence the new theory can be used to derive an estimate of the elastically supported differential stress in the slates at the time of boudinage development. An associated palaeostrain analysis is also performed using a new strain reversal method. The boudinage occurs just below the base of a major fold-thrust complex, essentially a large overfold of Namurian and Westphalian rocks emplaced at the end of the Carboniferous over Devonian and Lower Carboniferous slates with thin quartzites and limestones. We interpret the boudinage in the quartzite layers as a gravitational spreading response to the superincumbent overfold. The thin limestones, however, show no competence contrast with the surrounding slates and we have therefore used a limestone flow law to estimate the rate of vertical shortening beneath the overfold. Using our boudinage strain value, we then estimate that the overfold took between 0.75 and 1.5 Ma to evolve, probably during the Stephanian.